1. Field of the Invention
The present invention relates to a reflecting optical system and, more particularly, to a reflecting optical system constituting an afocal optical system.
2. Related Background Art
An example of the conventional optical system parts of which are comprised of reflecting surfaces is the telescope as illustrated in FIG. 1. This telescope is of a type called Cassegrain reflecting telescope, which is composed of a concave mirror 151, a convex mirror 152, and an eyepiece lens 155. This telescope is constructed in such an arrangement that the concave mirror 151 reflects parallel light 154 from an object at infinity into a converging beam on the object side, the convex mirror 152 disposed on the object side of the concave mirror 151 reflects the beam toward an observer to form an object image on an intermediate image plane 153, and then the object image is observed using the eyepiece lens 155 disposed behind the intermediate image plane 153.
In this way, the Cassegrain reflecting telescope has the optical system the total length of which is shortened by folding an optical path of a telephoto lens system with a long total length of lens comprised of refracting lenses by the two opposed reflecting mirrors.
Further, the above-stated telescope is arranged to receive a nearly parallel incident beam from the object at infinity and to emit a nearly parallel beam. The optical system of this type is normally called as an afocal optical system, which is used mainly in an observation optical system such as a viewfinder of telescope or camera.
In addition to the example of the afocal optical system having the imaging point in the optical system as in the above-stated telescope, there are afocal optical systems having no imaging point of real image in the optical system.
For example, Japanese Laid-open Patent Application No. 59-204817, U.S. Pat. No. 3,152,209, or the like discloses a wide converter lens, which is an example of the afocal optical system having no imaging point of real image in the optical system.
The simplest arrangement of the afocal optical system having no imaging point in the optical system is a so-called inverted Galilean telescope comprised of a combination of a negative lens with a positive lens as shown in FIG. 2.
In FIG. 2, reference numeral 163 designates a negative lens the focal length of which is -f.sub.0 (f.sub.0 is positive), and 164 a positive lens the focal length f.sub.E of which is f.sub.E =f.sub.0 +e. Here, e is a distance between the negative lens 163 and the positive lens 164.
The action of this optical system is next described. A parallel beam 161 from the object at infinity is first incident to the negative lens 163 disposed nearest to the object to be refracted by the negative lens 163, thereby forming a virtual image of the object at point A f.sub.0 apart from the negative lens 163 on the object side.
Then the beam refracted by the negative lens 163 is incident to the positive lens 164. Since the object point A at that time is coincident with the position of the front focal point of the positive lens 164, the positive lens 164 focuses the image at infinity. Accordingly, this optical system is an afocal optical system 162 arranged to receive the parallel beam incident thereto from the object at infinity and to emit the parallel beam and having no imaging point of real image in the optical system.
At this time, the angular magnification .gamma. of the afocal optical system 162 can be obtained as a ratio of absolute values of the focal lengths of the negative lens 163 and positive lens 164, i.e., .gamma.=f.sub.0 /f.sub.E. The ratio .gamma. in this case is smaller than 1.
The afocal optical system can change the angular magnification of optical system while emitting the parallel beam from the incident parallel beam in this way. Therefore, by locating the afocal optical system in front of an imaging optical system, the image magnification (focal length) of the imaging optical system can be changed without changing the position of the image plane of the imaging optical system alone.
FIG. 3 is an optical layout drawing in which the afocal optical system shown in FIG. 2 is disposed in front of the imaging optical system. In FIG. 3, the afocal optical system 162 of FIG. 2 is located in front of the imaging optical system 171 with the focal length f.sub.M, whereby the combined focal length f.sub.A becomes one obtained by multiplying the focal length f.sub.M of the imaging optical system 171 by the angular magnification .gamma. of the afocal optical system 162. That is, f.sub.A =.gamma..multidot.f.sub.M. In the case of FIG. 3, f.sub.A is smaller than f.sub.M and therefore, the afocal optical system 162 becomes a wide converter.
The telescope of FIG. 2 has the negative lens 163 disposed on the object side and the positive lens 164 disposed behind thereof, but another example may be of an arrangement of so-called Galilean telescope in which the positive lens is disposed on the object side and the negative lens is disposed behind thereof. Since in this case .gamma. is greater than 1, the telescope has a function of tele-converter.
Although the Cassegrain reflecting telescope has the shortened total length as compared with the case of the optical systems composed of only the refracting optical system, it is arranged to once form the object image and locate the eyepiece lens behind it, and therefore, the Cassegrain reflecting telescope has the relatively long total length among the afocal systems.
In contrast with it, the afocal optical systems having no imaging point of real image can be constructed with short total lengths like the inverted Galilean telescope or the Galilean telescope as described above.
Such afocal optical systems having no imaging point of real image in the optical system are used as converter lens systems such as the so-called wide converter or tele-converter in such a way that the afocal optical system of a desired magnification is mounted on the object side of the imaging optical system as shown in FIG. 3 to change the focal length of the imaging optical system by the magnification of afocal optical system, or used in cameras or the like as a finder optical system such as an Albada finder.
The afocal optical systems of this type, however, need at least two lens systems and normally have to include two or more lenses in order to further improve the performance and specifications.
In addition, most of the conventional afocal optical systems were composed of rotationally symmetric refracting lenses and reflecting surfaces, which imposed restriction on the degrees of freedom of arrangement of optical components in optical apparatus.
This situation will be described referring to FIG. 4A and FIG. 4B. FIG. 4A and FIG. 4B are explanatory drawings where a converter lens composed of conventional lenses rotationally symmetric is mounted on a camera. FIG. 4A is a front view thereof and FIG. 4B is a side view thereof.
In FIG. 4A and FIG. 4B, the converter lens 183 is mounted in front of the imaging optical system (master lens) 182 of camera 181.
In general, the so-called front converter lens mounted in front of the imaging optical system 182 is easy in correction for aberration of converter lens but on the other hand, the lens size thereof tends to be extremely greater than that of the imaging optical system. As a result, since the converter lens 183 with a large outer diameter was mounted to the camera 181, there occurred some cases that the converter lens covered a part of the optical path of another optical system built in the camera 181, for example, such as a finder system 184 or a photometry optical system 185.
There were some other cases in which the converter lens 183 did not cover another optical component directly but interrupted a part of optical path 191 of another optical system, for example, of the finder system 184, as shown in FIG. 4B.